Investment casting is an ancient process for forming metal parts by pouring molten metals or alloys into ceramic molds, referred to in the art as shells. The shells initially are formed around wax patterns, and after shelling the wax patterns are melted out leaving an internal void that is shaped and having detail of the desired part. The molten metal or alloy is poured into the internal void of the shell, then cools, thereby solidifying into the desired shape and detail of the part. After the metal or alloy has cooled and solidified, the shell is removed revealing the desired part.
Investment casting shells start out as a wax part that is matching the shape, size and detail of the desired article. The wax part is typically dipped into an aqueous ceramic slurry, coated with sand, and then dried before reiterating until the requisite thickness of the shell is obtained. The shell drying environment varies by foundry as temperature control, relative humidity management, and air movement are employed to manage and control the drying of the shell.
Critical care is taken in drying each dip layer from face coat until seal coat, as soak back from later dip coats effect the moisture level of the inner coats which may negatively impact shell strength. Adequate drying is necessary to ensure the strength of the shell and the integrity of detail of the desired article. A ‘too wet’ section of the interior shell substrate may cause soak back resulting in structural failure or loss of necessary article detail of the shell. A ‘too dry’ section of the interior shell may cause cracking or splitting resulting in failures during the article pouring process, or defects in the finished article.
After the shell building process is complete, wax is melted out of the shell leaving an internal void within the ceramic shell exacting the shape, size, and detail of the desired article. This wax removal may be performed via a number of methods including an autoclave or flash-fire dewax process. After the wax is melted out of the shell, the shells are inspected for structural failures, often times the noted failures are repaired before the shell is moved to the foundry, that is if the structural defects are discovered
Prior to pouring molten metal or alloy into the internal shell void, the shell is inspected, repaired as needed, and ultimately is placed into an oven. The purpose of this ‘oven’ step is to burnout any organic materials, form high temperature ceramic bonds and to heat the shell to a temperature which will allow the molten metal or alloy to completely fill-out the detail prior to solidifying. Once the shell is heat cured and maintained at the desired temperature for the requisite time, the molten metal or alloy is poured, filling the internal shell void completely.
The metal or alloy is allowed to cool within the shell taking on the shape and full detail of the original wax part. After adequate cooling, the shell is removed revealing a metal casting in exacting shape and detail as the wax part. The cast part is cut off the runner and processed to the required specifications.
During each of the described steps in the investment casting process a number of variables impact the subsequent steps that ultimately affect finished casting article quality. In an effort to assure quality control, repeatability, and effectuate an economy in production, it is critical to identify the key variables and establish operating protocols to manage the same.
Having good process control in the shell room is critical in making high quality consistent shells that produce desired casting parts. Some of key variables that need to be monitored and controlled include, but are not limited to pH, SiO2 content, % polymer, slurry viscosity, shell drying, shell room temperature and relative humidity. Variations in any of these can impact the quality of the finished part.
Shell drying is likely one of the most critical variables that has proved difficult to accurately monitor and therefore control. Historically, it has been challenging to determine a reliable method or apparatus that provides real time information regarding variations in the temperature and moisture level within the shell substrates during the dipping and drying process. As discussed above, inadequate drying can cause casting defects such as excess metal, finning or even foundry run-outs. Excessive drying can result in longer processing time, in addition to surface-related casting defects such as cracks and rat-tailing. Knowing the temperature and level of dryness at the interior shell surface or within the substrates of the shell itself would provide valuable insight as to how to adjust or fine tune the dipping and drying processes to maximize efficiencies while maintaining quality control.
Prior to this invention, shell moisture or dryness monitoring had four accepted methods; visual indicator, weight loss studies, temperature and conductivity. The first two methods provide information as to the average dryness of the shell in it's entirety, with minimal to no accurate target specific data. The latter methods, temperature and conductivity can provide data at a specific location of a shell, however, both methods are limited. Measuring temperature and then inferring an estimated dryness in the internal passages of a mold can result in erroneous data if there isn't sufficient air movement to remove the moisture-saturated air. Conductivity can be an effective method to measure moisture content of the mold in a specific location, however, it is difficult to locate the probes such that substrate specific or area specific information can be measured.
While each of the described methods and apparatus tests for general shell drying, not one of the previous methods or apparatus provide accurate, reliable and repeatable testing for the specific substrates and areas most difficult to dry, including the inner passage ways, blind holes and slots. Also, later dip iterations may cause excess moisture to be absorbed into the drying shell substrates, thereby wicking back towards the wax part, creating a hidden wet area within the inner shell substrates that if not dried adequately, will cause a failure or defect.
The prior art is replete with control apparatus and methods relating to the ceramic slurry viscosity, drying methods of the shell between slurry dips, robotic dipping apparatus controlled by microprocessor, wax formulas, and exterior shell monitoring to forecast internal shell dryness. However, nothing in the prior art teaches or suggests a method or apparatus for real time monitoring and data recordation of the environmental conditions at specific target location at the shell interior, or within the shell substrates during both the slurry dipping process or shell drying periods.